Each pair of same size/type/capacity should be put in the same coloured slots. Keeping to this allows it to run in dual channel mode for a slight performance increase. So if you have two sticks, put them in the same coloured slots.

On 64 bit, it'll give you access to the full capacity of the ram other than the 3-ish GB you probably have now as well as some potential processing bonus power. Particularly for older systems driver availability might be an issue though. Might be better to move to 64 if you later do a major upgrade.